1. Field of the Invention
The present invention relates to a crystallization apparatus, a crystallization method, a phase modulation element, a device and a display apparatus used to generate a crystallized semiconductor film by irradiating a polycrystal semiconductor film or an amorphous semiconductor film with laser beams having a predetermined light intensity.
2. Description of the Related Art
For example, a thin film transistor (TFT) used for, e.g., a switching element which controls a voltage applied to pixels in a liquid crystal display (LCD) is formed on an amorphous silicon layer or a polysilicon layer.
The polysilicon layer has a higher mobility of electrons or electron holes than that of the amorphous silicon layer. Therefore, when a transistor is formed on the polysilicon layer, a switching speed is increased and a response speed of a display is improved as compared with a case that a transistor is formed on the amorphous silicon layer.
Further, a peripheral LSI can be constituted of a thin film transistor. Furthermore, there is an advantage that a design margin of any other component can be reduced. Moreover, when peripheral circuits such as a driver circuit or a DAC are incorporated in a display, these peripheral circuits can be operated at a higher speed.
Although the polycrystal silicon is formed of an aggregation of crystal grains, it has a lower mobility of electrons or electron holes than that of single crystal silicon. Additionally, many thin film transistors formed on the polycrystal silicon have a problem of irregularities in crystal grain boundary number in a channel portion. Thus, in order to improve the mobility of electrons or electron holes and reduce irregularities in crystal grain boundary number in the channel portion, there has been proposed a crystallization method which generates crystallized silicon with a large grain size.
As this type of crystallization method, there is known a “phase control ELA (Excimer Laser Annealing) method) which generates a crystallized semiconductor film by irradiating a phase shifter approximated to a polycrystal semiconductor film or an amorphous semiconductor film in parallel with excimer laser beams. The detail of the phase control ELA method is disclosed in, e.g., “Surface Science Vol. 21, No. 5, pp. 278-287, 2000”.
In the phase control ELA method, a light intensity distribution with an inverse peak pattern (pattern that a light intensity is substantially zero at the center and the light intensity is suddenly increased toward the periphery) that a light intensity is substantially zero at a point corresponding to a phase shift portion of a phase shifter is generated, and a polycrystal semiconductor film or an amorphous semiconductor film is irradiated with light beams having this light intensity distribution with the inverse peak pattern. As a result, a temperature gradient is generated in a fusion area in accordance with the light intensity distribution, a crystal nucleus is formed at a part which is solidified first in accordance with a point where the light intensity is substantially zero, and a crystal grows in the lateral direction from the crystal nucleus toward the periphery (which will be referred to as a “lateral growth” or a “lateral-directional growth” hereinafter), thereby producing single crystal grains with a large particle size.
Additionally, Jpn. Pat. Appln. KOKAI Publication No. 2000-306859 discloses a technique to perform crystallization by irradiating a semiconductor film with light beams having a light intensity distribution with an inverse peak pattern generated through a phase shift mask (phase shifter).
Further, “Amplitude of Silicon Thin Film/Phase Control Excimer Laser Fusion Recrystallization Method—New 2-D Position-controlled Large Crystal Grain Forming Method-”, Inoue, Nakata and Matsumura, The institute of Electronics, Information and Communication Engineers Transactions Vol. J85-C, No. 8, pp. 624-629, August 2002 discloses a technique to perform crystallization by irradiating a semiconductor film with light beams having a light intensity including a concave pattern and an inverse pattern generated by combining a phase shifter and a light absorption distribution (see FIG. 3 and a relevant description).
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-306859, according to the technique to form a light intensity distribution with an inverse peak pattern by using a phase shifter, the light intensity distribution with the inverse peak pattern is formed at a part corresponding to a phase shift portion. However, since the light intensity is not linearly increased and an additional irregular distribution is generated around the light intensity distribution with the inverse peak pattern, the growth of the crystal is apt to be finished halfway.
It is to be noted that the obtained light intensity distribution may be possibly caused to approximate an ideal distribution by adjusting an angle distribution of illumination light beams relative to the phase shifter or designing an arrangement position of the phase shifter. However, that design cannot be analytically performed with a perspective, and it can be expected that very complicated design conditions are given even if the analytic design can be realized.
On the other hand, as disclosed in The institute of Electronics, Information and Communication Engineers Transactions, according to the technique in which the phase shifter is combined with the light absorption distribution, a light intensity distribution with the concave pattern and the inverse peak pattern for crystallization can be obtained. However, it is difficult to realize this distribution. That is, forming a film having a light absorption distribution which continuously varies is generally difficult. Further, light beams having a very strong intensity for crystallization is undesirable since it tends to generate a deterioration in a film material of a film having a light absorption distribution due to heat from light absorption or a chemical change when a film to be crystallized is irradiated.